Control rod drives are used to position control rods in boiling water reactors (BWRs) to control the fission rate and fission density, and to provide adequate excess negative reactivity to shutdown the reactor from any normal operating or accident condition at the most reactive time in core life. Each control rod drive is mounted vertically in a control rod drive housing which is welded to a stub tube, which in turn is welded to the bottom head of the reactor pressure vessel. De-mineralized water supplied by the control rod drive hydraulic system serves as the hydraulic fluid for control rod drive operation.
The control rod drive is a double-acting, mechanically latched hydraulic cylinder. The control rod drive is capable of inserting or withdrawing a control rod at a slow controlled rate for normal reactor operation and of providing rapid control rod insertion (scram) in the event of an emergency requiring rapid shutdown of the reactor. This is accomplished by sliding vertical displacement of an index tube to which the control rod is coupled.
Referring to FIG. 1A, a spud 22 at the top of the index tube 6 of the control rod drive engages and locks into a socket 12 at the bottom of the control rod 10. Once coupled, the control rod drive and control rod form an integral unit which must be manually uncoupled by specific procedures before a control rod drive or control rod may be removed from the reactor. Index tube 6 is a nitrided stainless steel tube threaded internally at its upper end. The spud 22 in turn is externally threaded for engaging the threads in the upper end of index tube 6. This connection is secured in place by means of a band 5 with tab locks.
As seen in FIG. 1A, the spud 22 has a plurality of spring fingers which extend vertically upward and which flex radially. The spring fingers permit the spud 22 to enter the mating socket 12 on the control rod 10. During a latching operation, a lock plug 14 enters spud 22 from socket 12. The lock plug 14 has a circular outer periphery of radius such that radially inward flexure of the spud spring fingers is blocked. This engagement prevents uncoupling of the spud and the socket.
In accordance with conventional practice, two control rod uncoupling mechanisms are provided. In the first uncoupling technique, the lock plug 14 is raised against the return force of a spring 16 by an actuating shaft 18 which extends through the center of the control rod velocity limiter 19 to an unlocking handle 20 (see FIG. 1B) in the shape of a D-ring. The top end of the actuating shaft is connected to an unlocking handle 20 in the shape of a D-ring. The actuating shaft 18 is lifted by manipulation of unlocking handle 20. The control rod 10, with lock plug 14 raised, may then be grappled and lifted from the control rod drive.
Alternatively, the lock plug 14 is raised from below to uncouple the control rod drive from below the reactor vessel. To accomplish this, a special tool is attached to the bottom of the control rod drive and used to raise the piston tube (not shown). This action in turn raises an uncoupling rod, lifting lock plug 14 so that spud 22 disengages from the control rod coupling socket 12. The uncoupling rod consists of a rod 2 and a tube 4, supported in the base of the spud at the upper end of the control rod drive, as seen in FIG. 1A. The rod 2 is welded to the flared end of tube 4 such that a predetermined distance exists between the top of rod 2 and the top end of spud 22. In addition to its function in uncoupling, rod 2 positions the control rod lock plug 12 such that it supports (i.e., opposes radially inward deflection of) the spud fingers when the control rod 10 and control rod drive are coupled.
In accordance with conventional practice, the unlocking handle 20 is lifted using an unlatching tool. This unlatching tool is also capable of removing the control rod from the reactor core once it is uncoupled. The unlatching tool comprises an actuator finger, which must be precisely located underneath the unlocking handle, and a grapple hook, which hooks under the handle (8 in FIG. 2A and 8' in FIG. 2B). In response to activation of a pneumatic cylinder, the actuator finger raises the unlocking handle to uncouple the control rod. In response to activation of a hoist, the grapple hook lifts the uncoupled control rod.
Recently, a new type of control rod with an increased overall length has been put into service (see FIG. 2A). The increased length changes the distance from the control rod handle to the unlocking handle, i.e., L.sub.E &gt;L.sub.s. Because the existing unlatching tool for standard handle control rods (one of which is shown in FIG. 2B) has a fixed-length arm that supports the unlatching mechanism, that tool is not able to uncouple any extended handle control rods that may reside in the reactor core.
Initially, a new unlatching tool for extended handle control rods, using a concept identical to the existing unlatching tool but with a longer arm, was designed and built. In application, both the original and new tools had to be available to uncouple all control rods. Also required was detailed knowledge of the location and type of control rods in the core. This dual-tool method is inefficient and time consuming because the tools must be switched back and forth. Thus, there is a need for a single unlatching tool capable of uncoupling and grappling both types of control rods that may be present in the reactor core.